This disclosure relates to a method for balancing the voltage of multiple series-connected electrochemical cells of an electrical storage system of a hybrid electric vehicle. The method comprises the step of discharging the electrical storage system before cell balancing. The disclosure also relates to a hybrid electric vehicle comprising an electrical storage system, wherein a battery management unit of the electrical storage system is configured to discharge the electrical storage system before cell balancing. The disclosure is particularly advantageous in the field of electrical storage systems for heavy hybrid vehicles, such as trucks, buses, construction vehicles, or the like.
The electrical storage systems in electric and hybrid electric vehicles commonly comprise battery packs of multiple series-connected electrochemical cells, typically hundreds of lithium cells. However, no two cells are identical—there are always slight differences in for example capacity, state of charge, and self-discharge rate. The capacity and lifetime of the weakest cell limit the capacity and lifetime of the battery pack as a whole. In order to be able to obtain more energy and greater lifetime from the battery pack, the voltage is periodically redistributed among the cells in order to bring them all to a common state of charge. This is commonly referred to as cell balancing. One known method for balancing, cells is to discharge individual cells of the battery pack over balancing resistors inside the battery, described in e.g. US 2011/0316520.
Cell balancing is preferably performed at a low state of charge level, since a difference in cell state of charge at low state of charge levels corresponds to greater difference in the open circuit voltage of the cells.
In order to limit the temperature increase, the discharge power has to be low which in turn leads to an overall long battery cell balancing time.
There is thus a need for an improved battery balancing method removing the above mentioned disadvantage.
It is desirable to provide a method for balancing the voltage of multiple series-connected electrochemical cells of an electrical storage system of a hybrid electric vehicle method where the previously mentioned problem is at least partly avoided.
The disclosure concerns a method for balancing the voltage of multiple series-connected electrochemical cells of an electrical storage system of a hybrid electric vehicle. The method comprises discharging the electrical storage system by operating at least one large electrical machine of the vehicle at vehicle standstill until the state of charge of the electrical storage system or the cell having the lowest state of charge has reached a predetermined level, and subsequently balancing the voltage of the cells.
The disclosure further concerns a corresponding computer program, a corresponding computer program product, a corresponding computer system for implementing the method, and a corresponding hybrid electric vehicle comprising an electrical storage system.
By discharging the electrical storage system, prior to balancing, by operating at least one large electrical machine of the vehicle, the discharge power can be kept high while avoiding the heat release that would arise within the energy storage unit if discharge of the electrical storage system would be performed by means of internal balancing resistors. This leads to considerably shorter discharge times, such that the overall battery cell balancing time can be reduced.
The at least one large electrical machine may have a maximal output power of more than 1 kW, preferably more than 5 kW, and still more preferably more than 20 kW. The more power consuming electrical machine that is used for the discharge, the higher discharge power is obtained, and thus increased discharge rate.
The at least one large electrical machine may be an electrical traction machine or the main electrical generator of the hybrid electric vehicle. These electrical machines normally represent the most powerful electrical machines on a hybrid electric vehicle, and consequently have the potential to enable the fastest discharge rate of the electrical storage system when operated. The at least one large electrical machine may alternatively be any of an electric machine driving a vehicle and/or cargo air conditioning system, an electrical, machine driving an air compressor unit, an electrical machine driving a cooling fan, or an electrical machine driving a hydraulic pump of a hydraulic system. These electrical machines are normally less powerful than the electrical traction machine or the main electrical generator of the hybrid electric vehicle, but have instead the advantage of enabling accumulation of the discharged electrical energy. Accumulation, i.e. recuperation, of the discharged electrical energy from the electrical storage system may for example be realised by lowering/increasing the temperature of the cargo compartment a driving cabin, or charging the air storage container with compressed air, or charging a hydraulic accumulator. In this way, energy that would otherwise be wasted can come to use and replace energy that otherwise would have had to be generated.
Discharge of the electrical storage system (ESS) may be realised by setting a combustion engine of the hybrid electric vehicle in a non-combustion mode and rotating a crankshaft of the combustion engine by means of the electrical traction machine or the main electrical generator, depending on the powertrain layout of the hybrid electric vehicle. The electrical traction machine or the main electrical generator are nearly always the largest electrical machines in a hybrid electric vehicle, and using any of them for driving the crankshaft of the combustion engine in a non-combustion mode, i.e. cranking, is one of the most power requiring activities in a vehicle. Thus a high discharge rate of the electrical storage system can be obtained. The method may additionally comprise actuating an exhaust brake and/or an engine compression brake for increasing the torque required to rotate the crankshaft and thereby increasing the discharge rate of the electrical storage system.
The electrical storage system may comprise a cooling system that is operated during the discharge of the electrical storage system. A high discharge rate generally results in high beat generation from the electrical storage system. In many vehicles, the cooling system of the electrical storage system is operated by a separate electrical motor driving a pump for circulating a heat conducting liquid, such as water. Alternatively, the pump may be mechanically powered via the crankshaft when the engine turning. With active control of the temperature of the electrical storage system, the electrical storage system can be discharged much faster than with previously known methods without risking possibly harmful temperature increases in the electrical storage system.
The predetermined state of charge, at which the discharge should be ended, may be considered reached when the state of charge of the electrical storage system is less than 50%, preferably less than 40%, and still more preferably less than 35%. The aim of the discharge to a low SOC is to attain a larger variation in open circuit voltage (OCV) of each cell, thereby enabling increased accuracy in individual cell SOC determination. The cell OCV/SOC characteristic varies depending on the type of battery cell, and the OCV/SOC curve is relatively flat in a central region, and exhibits a larger inclination (dOCV/dSOC) at the end regions, i.e. at a large SOC and a low SOC.
The predetermined state of charge may be considered reached when the state of charge of the cell having the lowest state of charge is less than 30%, preferably less than 25%, and still more preferably less than 20%. The SOC of the ESS is generally estimated using coulomb counting in combination with periodical calibration. ESS SOC estimation may also be based on ESS open circuit voltage. As a consequence, an ESS having a relatively large unbalance between the individual cells of the ESS may according to this ESS SOC determination method become over discharged and thus permanently damaged even if the estimated ESS SOC is within an acceptable SOC interval. A predetermined cell low voltage threshold may thus advantageously be implemented, and discharge of the ESS for attaining a low SOC should be limited to prevent any cell from reaching a SOC below the low voltage threshold.
The predetermined state of charge may be considered reached when the derivative of the present electrical storage system output voltage with respect to the present electrical storage system state of charge dOCV/dSOC is more than two times higher than a minimum derivative of the electrical storage system output voltage with respect to the electrical storage system state of charge, preferably more than three times higher than said minimum derivative, and still more preferably more than four times higher than said minimum derivative. The dOCV/dSOC curve that describes the electrical storage system output voltage as a function of its state of charge is relatively flat, i.e. the derivative is very small, for an intermediate interval of state of charge. But below a certain state of charge level, the curve generally becomes steeper and steeper. Comparing the present derivative to the minimum derivative can thus provide an indication of the present state of charge level and thus also an indication when a sufficiently low SOC has been reached.
The predetermined state of charge may be considered reached when the derivative of the present output voltage of the cell having the lowest output voltage with respect to the present state of charge of said cell is more than five times higher than a minimum derivative of the output voltage of said cell with respect to the state of charge of said cell, preferably more than seven times higher than said minimum derivative, and still more preferably more than ten times higher than said minimum derivative. The dOCV/dSOC curve that describes the cell output voltage with respect to its state of charge is similar to that of the electrical storage system described in the previous paragraph. Also for the cell, the derivative of its output voltage with respect to its state of charge can give an indication of the present state of charge level of the cell.
The method may further comprise discharging the electrical storage system by simultaneously operating at least one additional electrical consumer of the vehicle, wherein the additional electrical consumer is any of an electrical heating radiator or an electrical power resistor system configured as a current sink. The additional electrical consumer contributes to increasing the discharge rate of the electrical storage system.
The method may also comprise simultaneously discharging of the electrical storage system by redistribution of electrical charge to another electrical storage system. This is also a way of recuperating the discharged electrical energy.
The method may also comprise the initial step of checking if the state of charge of the electrical storage system or the cell having the lowest state of charge is equal to or below a predetermined level, and omitting the step of discharging the electrical storage system by operating at least one large electrical machine of the vehicle if the state of charge of the electrical storage system or the cell having the lowest state of charge is equal to or below the predetermined level. If the state of charge of the electrical storage system or the cell having the lowest state of charge is equal to or below a predetermined level, there is no need for discharging the electrical storage system prior to balancing.
The hybrid electric vehicle may comprise an electrical traction machine or electrical generator having a maximal power output of more than 100 kW, preferably more than 150 kW. The hybrid electric vehicle may also have a weight of more than 8 tonnes, and preferably more than 16 tonnes. Heavy hybrid electric vehicles usually comprise high power electric components where the power variations are big and fast and the space for components is small. Therefore, these vehicles have a particular need for cell balancing.
The electrical storage system may comprise at least 100 series-connected battery cells, and preferably at least 150 series-connected cells. Many series-connected cells can store and deliver more energy that few series-connected cells. But with a greater number of cells connected in series, the more important is the cell balancing. If one weak cell prematurely runs out of charge cycles due to overcharging or excessive discharging, the entire pack of series-connected cells will have to be repaired or replaced.